The ignition of such an air-fuel mixture using a so-called spark plug represents a usual component of internal combustion engines for motor vehicles. In these ignition systems installed these days, the spark plug is supplied inductively, using an ignition coil, with a sufficiently high electric voltage so that an ignition spark at the end of the spark plug forms in the combustion chamber of the internal combustion engine in order to start the combustion of the air-fuel mixture.
During the operation of this customary spark plug, voltages up to more than thirty kilovolt may appear, residues such as soot, oil or coal as well as ashes from fuel and oil appearing, which, under certain thermal conditions are electrically conductive. However, at these high voltages, no sparkover or breakdowns may occur at the insulator of the spark plug, so that the electrical resistance of the insulator should not change even at the high temperatures that appear during the service life of the spark plug.
From German Patent No. DE 198 52 652, for example, a spark device is known in which the ignition of such an air-fuel mixture is undertaken in an internal combustion engine of a motor vehicle, using a coaxial line resonator. In this connection the ignition coil is replaced by a sufficiently powerful microwave source, for instance, a combination of a high frequency generator and an amplifier. In the case of a geometrically optimized coaxial line resonator, the field strength required for ignition then comes about at the open end of the plug-like line resonator, and an ignitable plasma path forms between the electrodes of the plug.
The electrical excitation of this known coaxial line resonator takes place by a lateral coupling, these feeding devices, to be sure, taking up an undefined angular position after the screwing in of the so-called HF plug. In order to convert the contact position to a better manageable axial position, possibly by appropriately constructive measures, a relatively large radial or even axial space requirement consequently becomes necessary even when screwing it in.
Such a high frequency ignition is also described in general in the paper “SAE Paper 970071, Investigation of a Radio Frequency Plasma Ignitor for Possible Internal Combustion Engine Use”. In the case of this high frequency ignition or microwave ignition, without the usual ignition coil but using low-ohm feeding, a high voltage is generated at the so-called hot end of a λ/4 line of an HF line resonator.